These studies represent progress on a long range research program designed to improve clinical diagnosis of instability in the human spine. Decisions about the presence and absence of instability in the human spine are very difficult to make and there is little in the way of criteria guidelines for making that determination objectively. The decision is important, both in terms of risk for the patient and in terms of the overall utilization of the healthcare system. The decision will oftentimes determine whether the patient is treated with a surgical procedure or in a conservative non-surgical manner. In general these studies have been done on fresh autopsy material in which the physiologic conditions of the in vivo spine are simulated in vitro. The overall plan of the experiment has been to load the spine in a way to simulate either flexion or extension and sequentially destroy the various elements of the spine going from front to back. By carefully measuring the amount of linear displacement (sagittal plane translation) and rotary displacement (sagittal plane rotation) it has been possible to develop criteria of measurement which will offer an objective indication of the stability of the units of the spine. Work has been completed on the lower cervical spine and also on the thoracic spine. Work is now in progress on the lumbar spine and the lumbo-sacral spine. The previously determined objective criteria have been incorporated into a checklist has certain checks and balances and also incorporates clinical experience and other criteria to upgrade the validity of the ultimate decision. Similarly such a checklist has been developed for the thoracic spine; and this is now in process for the lumbar spine. In addition to the clinically relevant aspects of these studies there have also been more basic studies of the mechanical properties of the motion segments in the spine. The stiffness and flexibility characteristics have been developed for the different regions.